AutoValue Release 1.3-rc2

Bugs fixed

• The constructor of an @autovalue class that has a builder needs to have null checks if there are extensions, because in that case the constructor is package-private rather than private. Fixes #345.
• When scanning a Builder for referenced types, don't include non-abstract methods. The referenced types are used to determine the imports that will be needed by the implementing class, and that class never references non-abstract methods.